1. Field of the Invention
The present invention relates to a method of manufacturing a birefringent film, as well as an optical film using the same and a variety of image display devices.
2. Discussion of the Background
For the purpose of improving the image display quality such as through compensation for the hue coloring or widening the viewing angle in a variety of image display devices such as a liquid crystal display (LCD) device, a birefringent film is generally used. The birefringent film used for such a purpose is manufactured by using a polymer film, which is subjected to a uniaxial or biaxial stretching process to have its birefringence and retardation adjusted.
When using the thus manufactured birefringent film along with a polarizing film in an LCD device, these films are required to be arranged to have a slow axis of the birefringent film oriented substantially at right angles to an absorption axis of the polarizing film. In general, the slow axis of the birefringence film corresponds to its stretching direction, while the absorption axis of the polarizing film corresponds to its stretching direction.
Meanwhile, when a polymer film is industrially stretched, it is general to have a lengthwise polymer film subjected to the stretching treatment, while moving the same in the lengthwise direction so as to have it rolled up. When the thus rolled birefringent film and the rolled polarizing film are to be laminated together, they are respectively fed from the rolls so as to have their lateral sides (longitudinal edges) respectively positioned parallel to each other, and successively laminated together (so called roll to roll lamination). For this operation, it is necessary to stretch the polarizing film in the lengthwise direction, while stretching the birefringent film in the widthwise direction.
However, the following problem may arise during the manufacturing by arranging the birefringent film and the polarizing film in the above manner. For example, in manufacturing the birefringent film, a so-called bowing phenomenon is likely to be caused when successively stretching a polymer film in the widthwise direction, which phenomenon skews the in-plane orientation axis to the form of a bow. Therefore, it is difficult to uniformly cause the orientation axis, the birefringence and the retardation to a film by the stretching in the widthwise direction.
In light of the above, a technique of stretching a polymer film in the widthwise direction as employed includes applying a relaxing treatment to a polymer film after stretching in the widthwise direction so as to relax it back to an original width, thereby improving the uniformity of the slow axis with respect to the widthwise (stretching) direction. According to this method, a polymer film, which has been laminated on a substrate, is extended along with the substrate and then the substrate is relaxed (loosened) back to the original width so as to shrink the film in the relaxing treatment. A problem is that some types of polymer film produce only a small relaxing effect and therefore it is difficult to efficiently manufacture a birefringent film in an industrially stable manner. Although an attempt was made to reduce the bowing phenomenon by cooling after stretching treatment, this phenomenon cannot be completely avoided, as described such as in a nonpatent literature (T. Yamada et al. Intn. Polym. Process., Vo. X, Issue 4, 334-340 (1995)).
According to another technique to overcome the above problem, a substrate with a polymer film laminated thereon is relaxed between chucks of a tenter stretching machine, and the substrate is subjected to heat treatment, as disclosed such as in Japanese Patent Application Publication No. Hei. 06-51116. This technique causes problems that the substrate is limited to a material of the type that has a heat shrinkability, and a substrate, particularly a substrate having an excessively large thickness, may be wrinkled after the relaxing treatment, thereby causing uneven birefringence in the polymer film. Also, other techniques, which focus on adjustment of the stretching conditions, include a technique of setting the width of a stretching film as a square root of the stretching ratio (as disclosed such as in Japanese Patent Application Publication No. Hei. 03-23405), a technique of regulating the shrinking range resulting from the lengthwise stretching (as disclosed such as in Japanese Patent Application Publication No. Hei. 02-191904), and a technique of applying thermal relaxation after the stretching (as disclosed such as in Japanese Patent Application Publication No. Hei. 05-249316).
In consideration of the above problems, it is an object of the present invention to provide a method of manufacturing a birefringent film that is capable of manufacturing a birefringent film with a high production efficiency, while omitting the bowing phenomenon when manufacturing a birefringent film by stretching a polymer film in the widthwise direction, thereby limiting unevenness in birefringence, retardation and orientation angle and hence producing excellent optical characteristics, as well as achieving successive lamination (so-called roll to roll lamination) when laminating with a polarizing film.